U.S. patent number 7,482,296 [Application Number 11/015,774] was granted by the patent office on 2009-01-27 for aluminoborosilicate glass and method for the production of crystallite-free gradient index lenses.
This patent grant is currently assigned to Grintech GmbH. Invention is credited to Sandra Hornschuh, Bernhard Messerschmidt, Torsten Possner, Ulf Possner, Christian Ruessel.
United States Patent |
7,482,296 |
Messerschmidt , et
al. |
January 27, 2009 |
Aluminoborosilicate glass and method for the production of
crystallite-free gradient index lenses
Abstract
The invention is directed to an aluminoborosilicate glass
containing alkali metals for the production of optical components
with refractive index gradients which are generated by ion exchange
of monovalent metal ions in a base glass comprising at least
silicon oxide, boron oxide, aluminum oxide and an alkali metal
oxide, particularly for the production of gradient index lenses
(GRIN lenses). The object of the invention, to find a novel
possibility for producing GRIN lenses based on aluminoborosilicate
glasses in which the glass has an appreciably reduced tendency
toward crystallization in subsequent thermal treatment processes,
is met according to the invention in an alkali-containing
aluminoborosilicate glass for the production of optical components
with refractive index gradients generated by ion exchange of
monovalent, refractive index-changing metal ions in a base glass of
SiO.sub.2, Al.sub.2O.sub.3, a metal-(III)-oxide (of B and/or Ga)
and at least one metal-(I)-oxide (of Li, Na, K and/or Rb) in that
defined molar ratios of metal-(III)-oxides to the metal-(I)-oxides
of the base glass are adjusted within a given range in order to
appreciably reduce the tendency toward crystallization of the glass
for subsequent thermal treatment processes. Point defects caused by
crystallites in the GRIN lenses produced in this manner are
drastically reduced in this way.
Inventors: |
Messerschmidt; Bernhard (Jena,
DE), Ruessel; Christian (Jena-Cospeda, DE),
Hornschuh; Sandra (Jena, DE), Possner; Torsten
(Jena, DE), Possner; Ulf (Jena, DE) |
Assignee: |
Grintech GmbH (Jena,
DE)
|
Family
ID: |
34485604 |
Appl.
No.: |
11/015,774 |
Filed: |
December 17, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050137075 A1 |
Jun 23, 2005 |
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Foreign Application Priority Data
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Dec 19, 2003 [DE] |
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103 61 555 |
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Current U.S.
Class: |
501/77;
359/196.1; 501/13; 501/73; 501/79; 65/30.13 |
Current CPC
Class: |
C03C
3/064 (20130101); C03C 3/091 (20130101); C03C
3/118 (20130101); C03C 21/002 (20130101); C03C
21/005 (20130101) |
Current International
Class: |
C03C
4/00 (20060101); C03C 15/00 (20060101); C03C
19/00 (20060101); C03C 3/062 (20060101); G02B
26/08 (20060101); C03C 3/064 (20060101); C03C
3/066 (20060101) |
Field of
Search: |
;501/13,73,77,79 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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269 615 |
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Sep 1985 |
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DE |
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38 03 420 |
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Aug 1989 |
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DE |
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102 09 612 |
|
Oct 2002 |
|
DE |
|
0 918 235 |
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May 1999 |
|
EP |
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1 106 586 |
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Jun 2001 |
|
EP |
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WO 02/14233 |
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Feb 2002 |
|
WO |
|
Other References
Derwent Abstract 1989-242292, English Abstract of DE 38 03 420 A1.
cited by examiner .
Journal of Non-Crystalline Solids 347 (2004) pp. 121-127 "Silver
ion exchange in glasses of the system
Na.sub.2O/Al.sub.2O.sub.3/B.sub.2O.sub.3/SiO.sub.2" Hornschuh, et
al. cited by other.
|
Primary Examiner: Lorengo; J. A.
Assistant Examiner: Bolden; Elizabeth A
Attorney, Agent or Firm: Reed Smith LLP
Claims
What is claimed is:
1. Alkali-containing aluminoborosilicate glass for the production
of optical components with refractive index gradients which are
produced through ion exchange of monovalent metal ions in a base
glass of the following composition: 20.ltoreq.SiO.sub.2.ltoreq.50
mole % 10.ltoreq.B.sub.2O.sub.3.ltoreq.20 mole %
10.ltoreq.Al.sub.2O.sub.3.ltoreq.35 mole %
10.ltoreq.M.sub.2O.ltoreq.34 mole % (M=Li, Na, K, Rb), wherein a
molar ratio of <.times..times.< ##EQU00007## is adjusted in
order to reduce the tendency of the base glass toward
crystallization for subsequent thermal treatment processes.
2. Aluminoborosilicate glass according to claim 1, wherein a molar
ratio of .ltoreq..times..times..ltoreq. ##EQU00008## is adjusted in
the melt of the base glass.
3. Aluminoborosilicate glass according to claim 1, wherein the base
glass contains between 15 and 30 mole % of M.sub.2O, preferably
Na.sub.2O.
4. Aluminoborosilicate glass according to claim 1, wherein the base
glass further comprises Ag.sub.2O in a concentration of up to 2
mole %.
5. Aluminoborosilicate glass according to claim 1, wherein lithium
oxide has a proportion between 10% and 80% of the monovalent metal
oxides.
6. Aluminoborosilicate glass according to claim 1, wherein an
alkali fluoride or at least one oxide or fluoride of the metals
zinc, magnesium, calcium, strontium, barium, niobium and tantalum
are contained as admixtures in the base glass in concentrations of
up to 3 mole %, respectively.
7. Alkali-containing aluminoborosilicate glass for the production
of optical components with refractive index gradients which are
produced through ion exchange of monovalent metal ions in a base
glass of the following composition: 20.ltoreq.SiO.sub.2.ltoreq.50
mole % 10.ltoreq.M'''.sub.2O.sub.3.ltoreq.20 mole % (M'''=B, Ga)
10.ltoreq.Al.sub.2O.sub.3.ltoreq.35 mole %
10.ltoreq.M.sub.2O.ltoreq.34 mole % (M=Li, Na, K, Rb), wherein a
molar ratio of <.times.'''.times..times.< ##EQU00009## is
adjusted in order to reduce the tendency of the base glass toward
crystallization for subsequent thermal treatment processes.
8. Aluminoborosilicate glass according to claim 7, wherein a molar
ratio of .ltoreq..times..times..ltoreq. ##EQU00010## is adjusted in
the melt of the base glass.
9. Aluminoborosilicate glass according to claim 7, wherein the base
glass contains between 15 and 30 mole % of M.sub.2O, preferably
Na.sub.2O.
10. Aluminoborosilicate glass according to claim 7, wherein the
base glass further comprises Ag.sub.2O in a concentration of up to
2 mole %.
11. Aluminoborosilicate glass according to claim 7, wherein lithium
oxide has a proportion between 10% and 80% of the monovalent metal
oxides.
12. Aluminoborosilicate glass according to claim 7, wherein an
alkali fluoride or at least one oxide or fluoride of the metals
zinc, magnesium, calcium, strontium, barium, niobium and tantalum
are contained as admixtures in the base glass in concentrations of
up to 3 mole %, respectively.
13. A method for producing crystallite-free GRIN lenses from the
aluminoborosilicate glass of claim 7, the method comprising:
melting the base glass of the indicated composition at temperatures
of 1400.degree. C.-1600.degree. C. and homogenizing the molten
glass; casting the molten glass for producing semifinished articles
for optical components; shaping the semifinished articles to form
blanks for the ion exchange, wherein the semifinished articles are
reduced to dimensions suitable for the ion exchange; and ion
exchange by introducing the blanks at least once into a molten salt
in order to adjust a desired refractive index profile.
14. The method according to claim 13, wherein the blanks are
produced by cutting plates from square semifinished articles of
base glass and subsequently grinding or polishing until achieving a
height that is equal to or slightly greater than the definitive
lens height perpendicular to the optical axis.
15. The method according to claim 13, wherein the blanks are
produced from bar-rod-shaped semifinished articles of the base
glass by rod drawing at a temperature between 800.degree. C. and
1000.degree. C. and subsequent cutting, grinding or polishing until
reaching a diameter that is equal to or slightly greater than the
definitive lens diameter perpendicular to the optical axis.
16. The method according to claim 13, wherein the blanks are
produced by cutting cubes from a square semifinished article of
base glass and subsequently grinding and polishing until achieving
a spherical shape whose diameter is equal to or slightly greater
than the definitive lens diameter.
17. The method according to claim 13, wherein the blanks are
produced directly from the melt of the base glass by means of a
float glass process at a temperature between 800.degree. C. and
1600.degree. C. at a height that is equal to or slightly greater
than the definitive lens height perpendicular to the optical
axis.
18. The method according to claim 13, wherein, after the
introduction of silver ions, the blanks are further produced by
cutting the blanks and subsequent grinding or polishing until the
desired final lens dimensions are achieved perpendicular to and in
the direction of the optical axis.
19. The method of claim 18, wherein said aluminoborosilicate
glasses are used as GRIN dispersive lenses.
20. The method of claim 18, wherein an exchange equilibrium was
achieved during the introduction of silver, said use being as
optical components with integrally increased refractive index
level.
21. The method of claim 18, wherein said aluminoborosilicate
glasses are used for beam shaping laser diodes.
22. The method of claim 18, wherein said aluminoborosilicate
glasses are used as GRIN lenses for fiber-optic arrangements in
communications technology or sensor technology.
23. The method of claim 18, wherein said aluminoborosilicate
glasses are used as GRIN lenses for imaging beam paths and
illumination beam paths in medical engineering, particularly in
endoscopy.
24. The method according to claim 13, wherein silver ions are
introduced in an ion exchange with alkali ions by introducing the
blanks in a silver-containing molten salt at temperatures between
200.degree. C. and 600.degree. C. until a desired refractive index
profile is adjusted.
25. The method according to claim 24, wherein, after the
introduction of silver, the blanks are subjected to at least one
additional ion exchange process in an alkali-containing melt in
which there is a partial back-exchange of silver ions with
monovalent metal ions, preferably sodium ions, after which the
blanks are finished by cutting plates and subsequently grinding or
polishing until achieving the desired definitive lens dimensions
perpendicular to and in direction of the optical axis.
26. The method according to claim 25, wherein said
aluminoborosilicate glasses are used as GRIN collecting lenses.
27. The method according to claim 25, wherein said
aluminoborosilicate glasses are used as GRIN lenses for beam
shaping of laser diodes.
28. The method according to claim 25, wherein said
aluminoborosilicate glasses are used as GRIN lenses for fiber-optic
arrangements in communications technology or sensor technology.
29. The method according to claim 25, wherein said
aluminoborosilicate glasses are used as GRIN lenses for imaging
beam paths and illumination beam paths in medical engineering,
particularly in endoscopy.
30. The method according to claim 13, wherein a high proportion of
lithium ions is introduced into the melt of the base glass as metal
oxide and a partial exchange of lithium ions with other monovalent
metal ions, preferably sodium ions, is carried out during the ion
exchange in an alkali-containing melt until a desired refractive
index profile is adjusted, and wherein the blanks are further
produced by subsequently reducing the diameter until the desired
definitive lens height is achieved perpendicular to the optical
axis and by cutting and polishing until the definitive lens
thickness is achieved in direction of the optical axis.
31. A method for producing crystallite-free GRIN lenses using an
aluminoborosilicate glass of the following composition:
20.ltoreq.SiO.sub.2.ltoreq.50 mole %
10.ltoreq.M'''.sub.2O.sub.3.ltoreq.20 mole % (M'''=B, Ga)
10.ltoreq.M.sub.2O.ltoreq.34 mole % (M=Li, Na, K, Rb)
10.ltoreq.Al.sub.2O.sub.3.ltoreq.35 mole % and adjustment of molar
ratios to
<.times..times.<.times..times..times..times.<.times.'''.times-
..times.< ##EQU00011## containing the following steps: melting a
base glass of the indicated composition at temperatures of
1400.degree. C.-1600.degree. C. and homogenizing the molten glass;
casting the molten glass for producing semifinished articles for
optical components; shaping the semifinished articles to form
blanks for the ion exchange, wherein the semifinished articles are
reduced to dimensions suitable for the ion exchange; and ion
exchange by introducing the blanks at least once into a molten salt
in order to adjust a desired refractive index profile.
32. The method according to claim 31, wherein the blanks are
produced by cutting plates from square semifinished articles of
base glass and subsequently grinding or polishing until achieving a
height that is equal to or slightly greater than the definitive
lens height perpendicular to the optical axis.
33. The method according to claim 31, wherein the blanks are
produced from bar-rod-shaped semifinished articles of the base
glass by rod drawing at a temperature between 800.degree. C. and
1000.degree. C. and subsequent cutting, grinding or polishing until
reaching a diameter that is equal to or slightly greater than the
definitive lens diameter perpendicular to the optical axis.
34. The method according to claim 31, wherein the blanks are
produced by cutting cubes from a square semifinished article of
base glass and subsequently grinding and polishing until achieving
a spherical shape whose diameter is equal to or slightly greater
than the definitive lens diameter.
35. The method according to claim 31, wherein the blanks are
produced directly from the melt of the base glass by means of a
float glass process at a temperature between 800.degree. C. and
1600.degree. C. at a height that is equal to or slightly greater
than the definitive lens height perpendicular to the optical
axis.
36. The method according to claim 31, wherein, after the
introduction of silver ions, the blanks are further produced by
cutting the blanks and subsequent grinding or polishing until the
desired final lens dimensions are achieved perpendicular to and in
the direction of the optical axis.
37. A method of using aluminoborosilicate glasses produced
according to claim 36, said use being as GRIN dispersive
lenses.
38. A method of using aluminoborosilicate glasses produced
according to claim 36, wherein an exchange equilibrium was achieved
during the introduction of silver, said use being as optical
components with integrally increased refractive index level.
39. A method of using aluminoborosilicate glasses produced
according to claim 36, said use being for beam shaping of laser
diodes.
40. A method of using aluminoborosilicate glasses produced
according to claim 36, said use being as GRIN lenses for
fiber-optic arrangements in communications technology or sensor
technology.
41. A method of using aluminoborosilicate glasses produced
according to claim 36, said use being as GRIN lenses for imaging
beam paths and illumination beam paths in medical engineering,
particularly in endoscopy.
42. The method according to claim 31, wherein silver ions are
introduced in an ion exchange with alkali ions by introducing the
blanks in a silver-containing molten salt at temperatures between
200.degree. C. and 600.degree. C. until a desired refractive index
profile is adjusted.
43. The method according to claim 42, wherein, after the
introduction of silver, the blanks are subjected to at least one
additional ion exchange process in an alkali-containing melt in
which there is a partial back-exchange of silver ions with
monovalent metal ions, preferably sodium ions, after which the
blanks are finished by cutting plates and subsequently grinding or
polishing until achieving the desired definitive lens dimensions
perpendicular to and in direction of the optical axis.
44. A method of using aluminoborosilicate glasses produced
according to claim 43, said use as GRIN collecting lenses.
45. A method of using aluminoborosilicate glasses produced
according to claim 43, said use as GRIN lenses for beam shaping of
laser diodes.
46. A method of using aluminoborosilicate glasses produced
according to claim 43, said use as GRIN lenses for fiber-optic
arrangements in communications technology or sensor technology.
47. A method of using aluminoborosilicate glasses produced
according to claim 43, said use as GRIN lenses for imaging beam
paths and illumination beam paths in medical engineering,
particularly in endoscopy.
48. The method according to claim 31, wherein a high proportion of
lithium ions is introduced into the melt of the base glass as metal
oxide and a partial exchange of lithium ions with other monovalent
metal ions, preferably sodium ions, is carried out during the ion
exchange in an alkali-containing melt until a desired refractive
index profile is adjusted, and wherein the blanks are further
produced by subsequently reducing the diameter until the desired
definitive lens height is achieved perpendicular to the optical
axis and by cutting and polishing until the definitive lens
thickness is achieved in direction of the optical axis.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority of German Application No. 103 61
555.5, filed Dec. 19, 2003, the complete disclosure of which is
hereby incorporated by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to an aluminoborosilicate glass
containing alkali metals and at least a partial volume of silver
ions that are introduced into a base glass of silicon oxide, boron
oxide, aluminum oxide and at least one alkali metal oxide by ion
exchange with alkali metal ions, particularly for gradient index
lenses (GRIN lenses), and to a method for the production of GRIN
lenses and other optical elements with an at least partially
increased or reduced index of refraction.
b) Description of the Related Art
Numerous solutions are known from the prior art for producing
gradient index lenses (GRIN lenses). The most effective solutions
are based on borosilicate glasses which are suitable for ion
exchange in order to increase or partially change the index of
refraction.
For example, U.S. Pat. No. 4,902,330 describes a method for the
production of GRIN lenses which is based on a two-step process of
ion exchange. In the first step, an exchange of alkali metal ions
is caused by introducing the starting glass into a melt with silver
ions, thallium ions or lithium ions in order to increase the index
of refraction uniformly. In the second phase, the introduction of
silver ions in the glass from the first phase is partially
cancelled through immersion in molten salt with sodium ions or
potassium ions in order to generate a gradient of the index of
refraction. This two-step process is described for phosphate
silicate glasses and borosilicate glasses. For borosilicate
glasses, an unwanted coloration by silver colloids is mentioned as
disadvantageous.
A fundamental principle for generating differences in the index of
refraction in glasses is described in DD 269 615 B5 which discloses
glass compositions which are distinguished in that large
differences in the index of refraction are achieved in that oxides
of trivalent metals (M'''.sub.2O.sub.3, where M'''=B, Al, Ga) and
oxides of monovalent metals (M'.sub.2O, where M'=Li, K, Na, Rb)
which are contained in the glass have a molar ratio of .gtoreq.1.0
and a glass of this kind is brought into contact with silver,
silver alloys, molten salt or solutions at temperature between
210.degree. C. and 450.degree. C. The coloring occurring in this
process is described as minor. However, the tendency towards
crystallization (nepheline) which occurs as a result of thermal
shaping processes (rod drawing or float glass process) and which is
noticeable as punctiform or chain defects in the GRIN lenses and
generates imaging defects or scattered light is
disadvantageous.
The teaching of U.S. Pat. No. 6,511,932 B2 is based on similar
glass compositions, wherein glass compositions expanded by an
obligatory addition of magnesium oxide (of up to 18 mole %) for the
ion exchange between silver and alkali is claimed with the
advantage of a reduced melt temperature. However, the addition of
MgO worsens transmission and contributes to an increased tendency
toward crystallization.
In U.S. Pat. No. 5,007,948, which relates to substantially
colorless glasses containing silver through ion exchange, the glass
has an atomic structure in which the proportion of non-bridging
oxygen atoms is less than 0.03 for purposes of preventing
coloration by the silver. It cannot be determined whether or not
the tendency toward crystallization is also sufficiently
reduced.
Further, for the production of GRIN lenses WO 02/14233 A1 describes
alkali-free borosilicate glasses with a high silver content in
which the high silver content (of at least two cation percent) is
present in the base glass by means of block melt without the
addition of alkali ions. The gradient of the index of refraction is
generated by subsequent ion exchange in alkali-containing molten
salt. It is not mentioned in, nor can it be determined from, this
reference whether or not visible defects such as those caused by
tendency toward crystallization in the glass, and so on, are
sufficiently rare or can be excluded.
All of the aforementioned references with ion exchange between
silver and alkali have the common drawback that point defects which
interfere with imaging systems and illumination systems occur in
the finished GRIN lenses when the base glass has a significant
tendency to form crystallites or has increased growth as soon as
thermal treatment processes (such as thermal shaping processes,
e.g., rod drawing or ion exchange processes) are applied to the
glass blanks that are solidified from the glass melt.
OBJECT AND SUMMARY OF THE INVENTION
It is a primary object of the invention to find a novel possibility
for producing GRIN lenses based on aluminoborosilicate glasses in
which the glass has an appreciably reduced tendency toward
crystallization without a deterioration of the positive
characteristics known for conventional borosilicate glasses, such
as meltability at temperatures conventionally used in glassmaking
(up to 600.degree. C.), with good optical quality and a high
refractive index gradient and without substantial coloration after
the introduction of silver.
According to the invention, this object is met by an
alkali-containing aluminoborosilicate glass containing in at least
a part of its volume silver ions that are introduced through ion
exchange with alkali ions in a base glass of the following
composition: 20.ltoreq.SiO.sub.2.ltoreq.50 mole %
10.ltoreq.Al.sub.2O.sub.3.ltoreq.35 mole %
10.ltoreq.B.sub.2O.sub.3.ltoreq.20 mole %
10.ltoreq.M.sub.2O.ltoreq.34 mole %, where M=Li, Na, K and/or Rb,
wherein a molar ratio of aluminum oxide to metal-(I)-oxide is
adjusted in the range of
<.times..times.< ##EQU00001##
The base glass preferably has a molar ratio of
.ltoreq..times..times..ltoreq. ##EQU00002##
In an alternative solution, the above-stated object is met by an
alkali-containing aluminoborosilicate glass containing in at least
a part of its volume silver ions that are introduced through ion
exchange with alkali ions in a base glass of the following
composition: 20.ltoreq.SiO.sub.2.ltoreq.50 mole %
10.ltoreq.Al.sub.2O.sub.3.ltoreq.35 mole %
10.ltoreq.M'''.sub.2O.sub.3.ltoreq.20 mole %, where M'''.sub.2=B
and/or Ga 10.ltoreq.M.sub.2O.ltoreq.34 mole %, where M=Li, Na, K
and/or Rb, wherein a molar ratio of metal-(III)-oxides to
metal-(I)-oxides is adjusted in the range of
<.times.'''.times..times.< ##EQU00003##
In addition to aluminum oxide, primarily boron and gallium can also
be contained as trivalent metal oxides.
In all of the variants mentioned above, the base glass advisably
contains between 15 and 30 mole % of M.sub.2O, preferably
Na.sub.2O. Further, it may be advisable when the molten base glass
already contains silver oxide in a concentration of up to 2 mole
%.
Further, the above-stated object is met through an
alkali-containing aluminoborosilicate glass of the following
composition: 20.ltoreq.SiO.sub.2.ltoreq.50 mole %
10.ltoreq.Al.sub.2O.sub.3.ltoreq.35 mole %
10.ltoreq.M'''.sub.2O.sub.3.ltoreq.20 mole %, where M'''=B, Ga
10.ltoreq.M.sub.2O.ltoreq.34 mole %, where M=Li, Na, K, Rb, wherein
the monovalent metal oxides have a proportion of lithium dioxide
between 10% and 80% and a molar ratio of metal-(III)-oxides to
metal-(I)-oxides is adjusted in the range of
<.times.'''.times..times.< ##EQU00004##
Alkali fluorides, one or more oxides and/or fluorides of the metals
zinc, magnesium, calcium, strontium, barium, niobium and tantalum
are advantageously melted in the base glass in concentrations of up
to 3 mole % as additional admixtures for all of the
aluminoborosilicate glasses described above.
Further, the object of the invention is met in a method for
producing GRIN lenses and other optical elements with an at least
partially increased index of refraction based on an
aluminoborosilicate glass described above by the following sequence
of steps: melting a base glass of the indicated composition at
temperatures of 1400.degree. C.-1600.degree. C. and homogenizing
the molten glass, casting the molten glass for producing
semifinished articles for optical components, shaping the
semifinished articles to form blanks for the ion exchange, wherein
the blanks are reduced to dimensions suitable for the ion exchange,
introducing silver ions by exchanging alkali ions by introducing
the blanks at least once into a silver-containing molten salt at
temperatures between 200.degree. C. and 600.degree. C. until a
desired refractive index profile is adjusted.
The blanks are advantageously produced by cutting plates from
square semifinished articles (bars of base glass) and subsequently
grinding or polishing until achieving a height that is equal to or
slightly greater than the definitive lens height perpendicular to
the optical axis. In order to produce rotationally symmetric lenses
it is advantageous to produce blanks from bar-shaped semifinished
articles (cylinders) of the base glass by bar drawing at a
temperature between 800.degree. C. and 1000.degree. C. and
subsequent cutting, grinding or polishing until reaching a diameter
that is equal to or slightly greater than the definitive lens
diameter perpendicular to the optical axis.
In order to fabricate GRIN spherical lenses, the blanks are
likewise advisably produced from a square semifinished article of
base glass by cutting cubes and subsequently grinding and polishing
until achieving a spherical shape whose diameter is equal to or
slightly greater than the definitive lens diameter.
Another advantageous way to produce blanks for ion exchange
consists in that the blanks are produced directly from the melt of
the base glass by means of a float glass process at a temperature
between 800.degree. C. and 1600.degree. C. at a plate height that
is equal to or slightly greater than the final lens height
perpendicular to the optical axis.
After the introduction of the silver ions, the blanks are
preferably finished by cutting and subsequent grinding or polishing
until the desired final lens dimensions are achieved perpendicular
to and in the direction of the optical axis. The
aluminoborosilicate glasses produced in this way are preferably
used as GRIN dispersive lenses, wherein the blanks are possibly
ground and polished to a suitable optical lens thickness in
direction of the optical axis. However, optical components can also
be formed during the introductions of silver such that their
refractive index profile deviates from that of a conventional GRIN
dispersive lens. The polished surfaces can be plane, concave or
convex.
The above method is supplemented after the introduction of silver
by at least one additional ion exchange process in an
alkali-containing melt for producing GRIN focusing lenses in which
there is a partial back-exchange of silver ions with monovalent
metal ions, preferably sodium ions, after which the blanks are
finished by cutting plates and subsequently grinding or polishing
until achieving the desired definitive lens dimensions
perpendicular to and in direction of the optical axis. The
back-exchange of silver can advisably be carried out from a state
of silver introduction (of the first ion exchange) which did not
result in a homogeneous exchange equilibrium with an integrated
increase in the level of the refractive index. However, this does
not exclude blanks with a homogeneous refractive index level which
were produced in the first ion exchange and which can also be used
for optical components with an integrated refractive index increase
as well as for the back-exchange of ions.
In another embodiment of the method for producing GRIN collecting
lenses, the introduction of silver ions (through ion exchange) is
replaced by the introduction of a substantial proportion of lithium
ions as alkali ions in the melt of the base glass in order to
increase the refractive index and the blanks generated from this
are subjected to at least one ion exchange process in which a
partial exchange of lithium ions with monovalent metal ions,
preferably sodium ions, is carried out in an alkali-containing
melt, after which the blanks are subsequently reduced in diameter
until the desired definitive lens height is achieved perpendicular
to the optical axis and by cutting and polishing until the
definitive lens thickness is achieved in direction of the optical
axis.
The aluminoborosilicate glasses generated by the above-mentioned
ion exchange steps Ag.sup.+/M.sup.+ or Li.sup.+/M.sup.+ (where
M.sup.+ is preferably Na.sup.+) are advantageously used as GRIN
collecting lenses or are formed as optical components in such a way
that their refractive index profile deviates from that of a
conventional GRIN collecting lens and the blanks are possibly
ground and polished to a suitable optical lens thickness in
direction of the optical axis. These polished surfaces can be
plane, concave or convex.
The divergent lenses or collecting lenses produced in accordance
with the method are preferably used for beam shaping in laser
diodes or as GRIN lenses for fiber-optic arrangements in
communications technology or sensor technology. Further, there are
advantageous applications as GRIN lenses for imaging ray paths and
illumination beam paths in endoscopy or generally for optical
scanning and imaging devices in medical engineering.
The core idea of the invention is based on the surprisingly
discovered fact that the formation of crystallization defects which
occurs in particular during thermal shaping processes (e.g., rod
drawing) but also during solidification from the melt and during
thermal ion exchange processes can be appreciably suppressed
compared to a conventional aluminoborosilicate glass (e.g.,
according to DD 269 615 B5) when a molar ratio of the
metal-(III)-oxides to metal-(I)-oxides, as was indicated above, or
at least the molar ratio between aluminum oxide and the
metal-(I)-oxides of the base glass is adjusted within a given
range. Particularly in the production of GRIN lenses by ion
exchange (preferably Ag/Na and Li/Na), this results in advantageous
optical components which achieve large differences in the index of
refraction (up to .DELTA.n=0.15) with high transmission (and low
coloration) in the visible and NTR spectral regions along with
mechanical and chemical stability.
The invention makes it possible to realize the production of GRIN
lenses based on aluminoborosilicate glasses in which the base
glass, particularly in the thermal treatment steps for generating
higher refractive indices and refractive index gradients, has an
appreciably reduced tendency toward crystallization while retaining
the positive characteristics of conventional borosilicate glasses
such as meltability at temperatures conventionally used in
glassmaking (up to 1600.degree. C.) with good optical quality and a
high index of refraction after the introduction of silver without
substantial coloration.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will be explained more fully in the following with
reference to a number of embodiment examples.
In a basic variant, the base glass according to the invention in
which silver ions are to be introduced in at least a part of its
volume through ion exchange with alkali ions is an
alkali-containing aluminoborosilicate glass of the following
composition: 20.ltoreq.SiO.sub.2.ltoreq.50 mole %
10.ltoreq.B.sub.2O.sub.3.ltoreq.20 mole %
10.ltoreq.Al.sub.2O.sub.3.ltoreq.35 mole %
10.ltoreq.M.sub.2O.ltoreq.34 mole % (M=Li, Na, K, Rb), where the
following molar ratio is to be maintained:
<.times..times.< ##EQU00005##
In a more general basic variant containing the same oxides, the
ratio is considered from the proportions (mole %) of all contained
oxides of trivalent metals with those of the monovalent metals and
is adjusted to the following range:
<.times..times..times.< ##EQU00006##
The method is shown first for the production of GRIN lenses and
other optical elements with a generally increased index of
refraction or refractive index gradient in its diverse possible
application-specific method steps in order to illustrate more
clearly the technical problem in the composition of
aluminoborosilicate glasses in relation to the object of the
invention.
A base glass of the composition described above (special glass
mixture for producing GRIN components as is described in more
detail in the following) is melted at temperatures between
1400.degree. C. and 1600.degree. C., homogenized and cast in
suitable shapes of semifinished articles (bars, rods or balls).
Depending on the type of optical components that are intended, ion
exchange blanks which, as finished blanks, already have a suitable
dimensioning of the optical element for the ion exchange are
produced from the solidified semifinished articles. The production
of the blanks for different specifications is carried out as
follows.
a) GRIN Cylindrical Lenses
Glass plates are sawed from bars of the base glass (semifinished
articles) and ground or polished in accordance with precision
optics to a height that is equal to or slightly greater than the
definitive lens height (perpendicular to the optical axis). Thermal
processes by which glass plates with quasi-polished surfaces can be
produced (e.g., float glass process) are also suitable for the
production of glass plates.
b) GRIN Rod Lenses i) Semifinished articles are fashioned as
cylindrical rods from the base glass after casting by grinding and
polishing. ii) These semifinished articles are (if necessary) made
into rod-shaped blanks having a diameter adapted to the ion
exchange process by means of a bar drawing process (similar to a
glass fiber drawing process) at temperatures between 800.degree. C.
and 1000.degree. C. This diameter conforms to the definitive lens
diameter or is slightly greater so that post-treatment is possible
after the ion exchange process.
The second process ii) is particularly critical with respect to
unwanted crystallite formation because the typical temperature
range for nucleation and growth processes of crystallites is
traversed from low to higher temperatures. This means that the base
glass which has cooled after the melting process is heated once
again and passes through temperature ranges at which the speed of
crystal nucleation is increased and, afterwards through at higher
temperature ranges in which the crystal rate is high. Accordingly,
particularly because of the thermal regimes of blank production by
means of bar drawings, the base glass is in danger of forming
crystallites that could appear as visible defects in the end
product. However, crystallites can already exist in the base glass
when the melt solidifies or can form or possibly grow in the
subsequent ion exchange process.
c) GRIN Spherical Lenses i) Blanks in the form of cubes are made
from the glass bars of the base glass in spherical shape by
grinding and polishing, or ii) the blanks are shaped by direct
shaping (e.g., drop method) from the melt and subsequently ground
and polished to form balls.
A) In a first ion exchange process, the specially made blanks are
immersed in silver-containing molten salt at temperatures between
200.degree. C. and 600.degree. C. In so doing, the M.sup.+ ions in
the glass (preferably Na.sup.+) are exchanged with silver ions from
the melt. The silver ion cause an increase in the index of
refraction. The ion exchange is carried out in accordance with the
laws of interdiffusion. A divergent lens effect occurs when a
parabolic-refractive index profile is generated with the minimum
index of refraction in the radial center of the rod or in the
middle of the cross section of the plate. When this ion exchange
process is carried out for a sufficient period of time, a global
increase in the refractive index occurs because a determined
exchange equilibrium between the exchange ions (e.g., Na.sup.+ and
Ag.sup.+) is adjusted homogeneously in the glass. However, for
special applications the introduction of silver is not continued up
to the exchange equilibrium.
B) A second ion exchange process comprising at least one step
(partial back-exchange of silver) must be carried out subsequent to
the first ion exchange process in order to generate collecting
lenses. For this purpose, the cylindrical bars or rectangular
plates form the silver ion melt are immersed again in
Na.sup.+-containing molten salt and there results--likewise in
accordance with the laws of interdiffusion--a partial back-exchange
of the silver ions in the glass through sodium ions from the melt.
When the process is conducted in a suitable manner, parabolic
profiles result with the maximum index of refraction in the radial
center of the bar rod or in the middle of the cross section of the
plate. This results in a collecting lens effect with ray path.
The bars or plates (slabs) from ion exchange process A or from both
exchange processes A and B are detached (sawed) in pieces of
suitable size and subsequently ground and polished on the end faces
in a plane-optical manner. This results in GRIN rod lenses or GRIN
cylindrical lenses with plane optical end faces. Convex or concave
end faces are also possibly produced in order to add a refractive
lens effect.
Different advantageous constructions for the base glass according
to the invention are described in the following first nine
examples. A commercial reference glass of the following
composition: 25 Na.sub.2O.25 Al.sub.2O.sub.3.12.5
B.sub.2O.sub.3.37.5 SiO.sub.2 (in mole %) is used to test the
crystallization tendency of the various embodiments of the base
glass.
EXAMPLE 1
A glass composed of 25 Na.sub.2O.25 Al.sub.2O.sub.3.12.5
B.sub.2O.sub.3.37.5 SiO.sub.2 (in mole %) was melted at
1500.degree. C. After temperature treatment for 1 hour at
temperatures .ltoreq.850.degree. C., no crystallization was
evident. At temperatures of 900.degree. C. and 950.degree. C., the
crystallization rate was about one fourth of the value of the
reference glass.
EXAMPLE 2
A glass composed of 25 Na.sub.2O.25 Al.sub.2O.sub.3.12.5
B.sub.2O.sub.3.37.5 SiO.sub.2 showed no discernible crystallization
in the temperature range of 700.degree. C. to 950.degree. C.
EXAMPLE 3
A glass composed of 24.5 Na.sub.2O.1 NaF.30 Al.sub.2O.sub.3.12.5
B.sub.2O.sub.3.32.5 SiO.sub.2 showed no crystallization at
temperatures .ltoreq.850.degree. C. With temperature treatment at
900.degree. C. and 950.degree. C. for 1 hour, the crystallization
rate reached values of about one fifth of the reference glass.
EXAMPLE 4
A glass composed of 25 Na.sub.2O.2.5 MgO.27.5 Al.sub.2O.sub.3.12.5
B.sub.2O.sub.3.32.5 SiO.sub.2 showed no crystallization when heated
in the range of 700.degree. C. to 950.degree. C.
EXAMPLE 5
A glass composed of 30 Na.sub.2O.32.5 Al.sub.2O.sub.3.15
B.sub.2O.sub.3.22 SiO.sub.2 showed no discernible crystallization
after heating at temperatures <800.degree. C. At temperatures
.gtoreq.850.degree. C., the crystallization rate was about one
fourth of the value of the reference glass.
EXAMPLE 6
A glass composed of 30 Na.sub.2O.30 Al.sub.2O.sub.3.20
B.sub.2O.sub.3.20 SiO.sub.2 showed no crystallization at
temperatures .ltoreq.850.degree. C. At 900.degree. C., the
crystallization rate was about one sixth of the value of the
reference glass. At 950.degree. C., the crystallization rate was
the same as that measured at 900.degree. C., i.e., an increase in
the crystallization rate with the temperature (as in the reference
glass) was not observed.
EXAMPLE 7
A glass composed of 25 Na.sub.2O.25 Al.sub.2O.sub.3.15
B.sub.2O.sub.3.35 SiO.sub.2 showed no crystallization at
temperatures .ltoreq.800.degree. C. At 850.degree. C., the
crystallization rate was about one third of the value of the
reference glass. At increased temperatures, the crystallization
rate dropped again and reached 1/40th of the value of the reference
glass at 950.degree. C.
EXAMPLE 8
A glass composed of 20 Na.sub.2O.22.5 Al.sub.2O.sub.3.12.5
B.sub.2O.sub.3.45 SiO.sub.2 showed no crystallization when heated
to temperatures up to 900.degree. C.
EXAMPLE 9
A glass composed of 25 Na.sub.2O.2.5 ZnO.27.5 Al.sub.2O.sub.3.12.5
B.sub.2O.sub.3.32.5 SiO.sub.2 showed no crystallization when heated
up to 850.degree. C. At higher temperatures, the crystallization
rate was about one fifth as high as that of the reference
glass.
EXAMPLE 10
A glass composed of 20 Na.sub.2O.5 Li.sub.2O.27.5
Al.sub.2O.sub.3.12.5 B.sub.2O.sub.3.35 SiO.sub.2 in which there was
no introduction of silver (ion exchange with alkali ions) to raise
the level of the refractive index but in which, instead, a high
proportion of lithium oxide (20%) was already incorporated in the
melt at the molar content of the monovalent metal oxides showed no
discernible crystallization after heating to temperatures
<750.degree. C. At temperatures .gtoreq.750.degree. C., the
crystallization rate was about one third of the value of a
reference glass which in this case had the composition 20
Na.sub.2O.5 Li.sub.2O.25 Al.sub.2O.sub.3.12.5 B.sub.2O.sub.3.37.5
SiO.sub.2.
EXAMPLE 11
A glass composed of 10 Na.sub.2O.15 Li.sub.2O.27.5
Al.sub.2O.sub.3.12.5 B.sub.2O.sub.3.35 SiO.sub.2 in which there was
no introduction of silver (ion exchange with alkali ions) to raise
the level of the refractive index but in which, instead, a high
proportion of lithium oxide (75%) is already incorporated in the
melt at the molar content of the monovalent metal oxides showed no
crystallization when heated up to 900.degree. C.
The scope of possible variations in the glass compositions and
therefore the degree of gradients of the index of refraction that
can be achieved by the ion exchange processes are in no way
exhausted by the described method using the aluminoborosilicate
glasses indicated above. Additional modifications of the
composition of the base glass by further slight additions of metal
oxides or metal fluorides which were not indicated herein or by
exceeding the indicated range limits are likewise comprehended in
the inventive teaching with the basic idea of the invention of
adjusting a defined surplus of trivalent metal oxides (particularly
aluminum oxide) in relation to the monovalent alkali metal oxides
in order to suppress the crystallization tendency of the glass in
the production of optical GRIN components.
While the foregoing description and drawings represent the present
invention, it will be obvious to those skilled in the art that
various changes may be made therein without departing from the true
spirit and scope of the present invention.
* * * * *